VirtualBox

VirtualBox is an interesting virtualization software where it basically allows you to run virtual PCs of multiple types of operating systems. If you’ve ever heard of VMware, this is basically the same thing. Built by Innotek, it basically consists of an open source version and an enterprise version. So far, the screenshots show that the maturity of the project seem to be very much on par with VMware Server, which is also free but not OSS.
DownloadSquad < VirtualBox